In the repair work on automobiles, for example, replacement of punctured tires, it is necessary to use a jack to raise the car body. Heretofore, a pantograph type manually operated screw jack has been used for such work. The pantograph type screw jack comprises a horizontally extending threaded rod, a base disposed below said threaded rod and adapted for placement on the ground, and a lifting table disposed above said threaded rod and adapted for engagement with a car body. The threaded rod has a threaded portion extending from one end toward the middle of said rod, with a thread-driven movable block screwed on said threaded portion. A parallelogram link mechanism is operatively associated with said elements to constitute a pantograph. The link mechanism is composed of four links forming a parallelogram, wherein the upper pair of links have one ends thereof pivotally connected to the lifting table and the other ends thereof pivotally connected respectively to the movable block on the threaded rod and to a fixed block rotatably attached to the other end of the threaded rod, while the lower pair of links have one ends thereof pivotally connected to the base and the other ends thereof pivotally connected respectively to said movable block and to said fixed block.
In the pantograph type screw jack, the threaded rod is rotated by manually operating a crank handle. With the rotation of the threaded rod, the movable block is threadedly moved lengthwise of the threaded rod toward the fixed block, so that the distance between the movable and fixed blocks is reduced. This reduction in the distance between the movable and fixed blocks results in the distance between the lifting table and the base being increased by the parallelogram link mechanism. Accordingly, by manipulating the handle to rotate the threaded rod, it is possible to raise the lifting table relative to the base placed on the ground, so as to raise the car body engaged by the lifting table.
A disadvantage of such pantograph type screw jack is that the amount or speed of upward travel of the lifting rod is not proportional to the amount or speed of rotation of the threaded rod. It is impossible for the parallelogram link mechanism to establish proportional relation between the rate of decrease in the distance between the movable and fixed blocks and the rate of increase in the distance between the lifting table and the base. Assuming that the rate of decrease in the distance between the movable and fixed blocks is constant, the rate of increase in the distance between the lifting table and the base will gradually lower as the distance between the lifting table and the base increases. Accordingly at the start of operation, namely, when the lifting table is positioned at a low level, manipulation of the handle to rotate the threaded rod will raise the lifting table at relatively high speed, but when the lifting table reaches a relatively high level, it will be raised only at low speeds. This means that at the start of operation, manipulation of the handle requires much force and that when the lifting table reaches a relatively high level, the efficiency of operation decreases.